RF-transistors with self-aligned point contacts

ABSTRACT

A method of fabricating a semiconductor device includes depositing a dielectric layer on a substrate and a nanomaterial on the dielectric layer. The method also includes depositing a thin metal layer on the nanomaterial and removing a portion of the thin metal layer from a gate area. The method also includes depositing a gate dielectric layer. The method also includes selectively removing the gate dielectric layer from a source contact region and a drain contact region. The method also includes patterning a gate electrode, a source electrode, and a drain electrode.

DOMESTIC PRIORITY

This application is a continuation of and claims priority from U.S.patent application Ser. No. 14/983,646, filed on Dec. 30, 2015, entitled“RF-TRANSISTORS WITH SELF-ALIGNED POINT CONTACTS”, the entire contentsof which are incorporated herein by reference.

BACKGROUND

The present invention relates to transistors, and more specifically, tohigh performance radio frequency transistors with improved resistanceproperties.

High performance radio frequency (RF) transistors are sensitive toparasitic resistances. Such parasitic resistances can arise, forexample, in a gate region of the (RF) transistors when a portion of gatedielectric region remains exposed after manufacture. Undesired parasiticresistances can significantly reduce the operation frequency of thedevice.

Carbon nanotubes (CNT) include carbon allotropes that are arranged in acylindrical nanostructure. Carbon nanotubes have unique semiconductingproperties that offer significant performance gains in manysemiconducting devices.

Carbon nanotubes may be fabricated using a variety of fabricationprocesses including, laser ablation, arc discharge, chemical vapordeposition, and plasma torch processes.

Carbon nanotubes, graphene, and related materials materials have highmobility, making them desirable candidates for RF transistors. However,manufacturing such devices is difficult with existing technologies dueto current process limitations. For example, it is difficult to stop atCNT and related materials in reactive ion etching (RIE). Thus,conventional processes for creating self aligned transistors cannot meetapplicable design criteria. There remains a need for high performance RFtransistors with increased mobility and reduced parasitic resistances.

SUMMARY

According to one embodiment of the disclosure, a method of fabricating asemiconductor device includes depositing a dielectric layer on asubstrate. The method also includes depositing a nanomaterial on thedielectric layer. The method also includes depositing a thin metal layeron the nanomaterial. The method also includes removing a portion of thethin metal layer from a gate area. The method also includes depositing agate dielectric layer. The method also includes selectively removing thegate dielectric layer from a source contact region and a drain contactregion. The method also includes patterning a gate electrode, a sourceelectrode, and a drain electrode.

According to another embodiment of the disclosure, a method of forming asemiconductor device includes depositing carbon nanotubes on asubstrate. The method also includes depositing a thin nickel layer onthe carbon nanotubes. The method also includes selectively removing thethin nickel layer from a gate area of the device. The method alsoincludes forming end bonded contacts between the carbon nanotubes andthe thin nickel layer in the gate area of the device.

According to yet another embodiment, a semiconductor device includes asemiconductor substrate. The semiconductor device also includes a thinmetal layer on the substrate. The semiconductor device also includescarbon nanotubes. The semiconductor device also includes a T-shapedgate. The semiconductor device also includes an electrode on thesubstrate and adjacent to the T-gate. In accordance with an embodiment,the carbon nanotubes have end-bonded contact to the thin metal layer onthe substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The forgoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 illustrates a cross sectional view of an exemplary RF transistorincorporating a conducting nanomaterial.

FIGS. 2A and 2B illustrate different types of CNT metal contacts, inwhich:

FIG. 2A illustrates a side-bonded CNT contact and FIG. 2B illustrates anend-bonded CNT contact.

FIG. 3 depicts a flow diagram of a method for preparing a transistor inaccordance with an exemplary embodiment of the disclosure.

FIGS. 4A-4H illustrate and exemplary method of fabricating a transistorin accordance with an embodiment of the disclosure, in which:

FIG. 4A is a cross sectional side view of an RF transistor afterdepositing a dielectric layer on a substrate according to an exemplaryembodiment;

FIG. 4B is a cross sectional side view of an RF transistor afterdepositing a nanomaterial on the dielectric layer according to anexemplary embodiment;

FIG. 4C is a cross sectional side view of an RF transistor afterdepositing a thin metal layer on the nanomaterial according to anexemplary embodiment;

FIG. 4D is a cross sectional side view of an RF transistor afterdepositing a resist layer on the thin metal layer, patterning the resistlayer, and wet etching to remove the thin metal layer from the gate areaaccording to an exemplary embodiment;

FIG. 4E is a cross sectional side view of an RF transistor afterdepositing a gate dielectric layer and etching the gate dielectric layerto expose the source and drain regions according to an exemplaryembodiment;

FIG. 4F is a cross sectional side view of an RF transistor afterdepositing and patterning a gate electrode, source electrode, and drainelectrode on the transistor according to an exemplary embodiment;

FIG. 4G is a cross sectional side view of an RF transistor afterconducting an optional low-temperature anneal according to an exemplaryembodiment; and

FIG. 4H is a cross sectional side view of an RF transistor afterremoving the resist layer from the transistor according to an exemplaryembodiment.

FIG. 5 is a chart illustrating a transfer curve of an RF transistoraccording to one embodiment of the disclosure.

DETAILED DESCRIPTION

The disclosure relates to fabrication of high performance RF transistorswith self-aligned point contacts. RF transistors are semiconductordevices can be used in a variety of applications.

Conventional methods of producing RF transistors with CNT and similar 2Dmaterials have suffered from high parasitic resistance. This can be due,for example, to constraints and limitations from conventional transistorfabrication methods, for example, due to scaling.

Conventional semiconductor fabrication methods can result in anundesirably large ungated region on the transistor, in which a portionof gate dielectric remains exposed in the gate area after semiconductorfabrication. This can result in a high parasitic resistance. Forexample, with respect to length, ungated regions can be 100 nanometers(nm) in length or more.

The semiconductor material under this ungated region cannot be modulatedby the gate electrode. Therefore, the semiconductor material under thisregion remains at high resistive state and introduces large parasiticresistance. The parasitic resistance can effectively reduce the actualbias voltage across source and drain electrodes. This can alsosignificantly reduce the device drive voltage, undesirably leading tomuch slower switching speed.

An illustration of an exemplary RF transistor incorporating a conductingnanomaterial is shown in FIG. 1. A dielectric layer 102 is deposited onsilicon substrate. A thin layer of conducting nanomaterial 104, such asCNT, or 2D materials such as graphene, can be deposited on thedielectric layer 102. The RF transistor contains a source electrode 106and a drain electrode 108.

A gate 110 can be positioned between the source electrode 106 and drainelectrode 108 and can be positioned on top of a gate dielectric 112.Ungated regions 114 adjacent to the gate 110 result from conventionalfabrication techniques and can result in high parasitic resistance.Reducing the size of these regions can be advantageous in highperformance applications. However, conventional semiconductor processesto create self aligned structures that might reduce the size of ungatedregions in other semiconductor systems, have limited applicability insystems using CNT and similar 2D materials. This can be due to, forexample, the difficulty of controlling the stopping point of reactiveion etching (RIE) when using such materials.

In transistor applications using carbon nanotubes, consideration can begiven to the type and/or quality of the CNT contact at the metalinterface. The quality of contacts between metal electrode and carbonnanotubes can impact transistor performance. FIGS. 2A and 2B illustratetwo different types of CNT-metal contacts.

FIG. 2A depicts a side bonded contact, in which a carbon nanotube 200contacts a metal 202 on the wall of the tube, or the tube side. Carbonnanotube 200 is a layer of cylindrical tubes formed from carbon atoms.Metal 202, in the case of transistors, can be metal of the source anddrain electrodes.

FIG. 2B illustrates a different type of contact between metal and carbonnanotubes, which is an end-bonded contact. In an end-bonded contact,metal 202 contacts the ends of the cylindrical carbon nanotube 200. Anend-bonded contact can be preferable in semiconductor applications, forexample, because it can form strong covalent bonds between carbon atomsand metal atoms. These bonds can enhance the carrier injection. Reducedresistance could be observed in structures including end-bonded contactscompared to structures including side-bonded contacts.

Referring now to FIG. 3, a flow diagram of a method for preparing atransistor in accordance with an exemplary embodiment is shown. Themethod 400 includes, as shown at block 402, depositing a dielectriclayer on a substrate. Next, as shown at block 404, the method 400includes depositing a nanomaterial on the dielectric layer. Then, asshown at block 408, a resist layer is deposited.

The method 400 also includes patterning the resist layer, as shown atblock 410. Then, as shown at block 412, thin metal layer is selectivelyremoved from the gate area. The method 400 also includes, as shown atblock 414, depositing a gate dielectric layer. Next, as shown at block416, source and drain contact regions are etched to remove the gatedielectric layer.

The method 400 also includes, as shown at block 418 depositing andpatterning a gate electrode and source and drain electrodes. The method400 optionally includes, as shown at block 420, performing a lowtemperature anneal. Then, as shown at block 422, the method 400 includesremoving the resist layer from above the thin metal layer.

Deposition is any process that grows, coats, or otherwise transfers amaterial onto the wafer. Available technologies include, but are notlimited to, thermal oxidation, physical vapor deposition (PVD), chemicalvapor deposition (CVD), electrochemical deposition (ECD), molecular beamepitaxy (MBE) and more recently, atomic layer deposition (ALD) amongothers.

Removal is any process that removes material from the wafer: examplesinclude etch processes (either wet or dry), and chemical-mechanicalplanarization (CMP), etc.

Patterning is the shaping or altering of deposited materials, and isgenerally referred to as lithography. For example, in conventionallithography, the wafer is coated with a chemical called a photoresist;then, a machine called a stepper focuses, aligns, and moves a mask,exposing select portions of the wafer below to short wavelength light;the exposed regions are washed away by a developer solution. Afteretching or other processing, the remaining photoresist is removed.Patterning also includes electron-beam lithography, nanoimprintlithography, and reactive ion etching.

FIGS. 4A-4H illustrate an exemplary method of fabricating a transistorin accordance with one embodiment of the disclosure. As shown in FIG.4A, a dielectric layer 502 is deposited on a substrate 500.

Substrate 500 can be a semiconductor substrate and can includesemiconducting material. The semiconducting material can include, but isnot limited to, Si (silicon), strained Si, SiC (silicon carbide), Ge(geranium), SiGe (silicon germanium), SiGeC (silicon-germanium-carbon),Si alloys, Ge alloys, GaAs (gallium arsenide), InAs (indium arsenide),InP (indium phosphide), or any combination thereof. In preferredembodiments, substrate 500 contains silicon.

The dielectric layer 502 can include any suitable dielectric material.In some embodiments, the dielectric layer 502 is a low-k gate dielectrichaving a dielectric constant less than 4. Non-limiting examples ofsuitable materials for the dielectric layer 502 include silicon dioxide,tetraethylorthosilicate (TEOS) oxide, carbon-doped oxides, siliconnitride, high aspect ratio plasma (HARP) oxide, high temperature oxide(HTO), high density plasma (HDP) oxide, oxides (e.g., silicon oxides,hafnium oxides) formed by an atomic layer deposition (ALD) process, orany combination thereof.

FIG. 4B illustrates the transistor according to an exemplary embodimentafter depositing a nanomaterial 504 on the dielectric layer 502.Nanomaterial 504 includes, for example, carbon nanotubes (CNTs),including highly purified CNTs and 2-dimensional (2D) materials, such asgraphene or transition metal dichalcogenide (TMDC) materials. In someembodiments, nanomaterials can be doped with other materials.

In some embodiments, the CNTs are highly purified CNTs. For example, insome embodiments, the semiconducting purity of CNT can be at least 90%.For example, in some embodiments, CNT is at least 95% pure. In someembodiments, CNT is at least 96% pure, or at least 97% pure, or at least98% pure, or at least 99% pure, or 100% pure. Methods of purifying CNTfor semiconductor applications are known.

As shown in FIG. 4C, a thin metal layer 506 is deposited on thenanomaterial 504. The thin metal layer 506 can include, for example,nickel (Ni), molybdenum (Mo), palladium (Pd), and other metals suitablein high performance transistors. In some embodiments, the thin metallayer includes nickel.

The thickness of the thin metal layer can be, for example, 5 nm to 50nm. In some embodiments, the thickness of the thin metal layer is 5 nmto 40 nm. In some embodiments, the thickness of the thin metal layer is5 nm to 30 nm. In some embodiments, the thickness of the thin metallayer is 5 nm to 20 nm. In some embodiments, the thickness of the thinmetal layer is 5 nm to 10 nm.

As illustrated in FIG. 4D, a resist layer 508 is deposited on the thinmetal layer 506 and patterned to expose the thin metal layer 506 in thesource region 510 and the drain region 512, the regions in which thesource and drain electrodes will be formed. Then, as illustrated, thethin metal layer is removed by wet etch in the gate area 514, exposingthe nanomaterial 504. If the metal layer is suitably thin, wet etch canbe conducted with minimal undercut.

Resist layer 508 can be a photoresist layer such as hydrogensilsesquioxane (HSQ). In preferred embodiments, resist layer 508 is alayer of HSQ. HSQ can be deposited on the thin metal layer and thenpatterned, for example, by first exposing portions of the HSQ layer(i.e., the portions of the HSQ layer that will remain after thepatterning) to an energy-yielding process that will cure and cross-linkthose portions of the HSQ layer.

For instance, HSQ layer can be cross-linked by exposing the HSQ layer toe-beam or extreme ultraviolet (EUV) radiation with wavelengths shorterthan 157 nanometers (nm). Next, unexposed portions of the HSQ layer canthen be selectively removed using a developer wash (such as aTetramethyl-ammonium hydroxide (TMAH) based developer or a salt baseddeveloper such as an aqueous mixture of sodium hydroxide (NaOH) alkaliand sodium chloride (NaCl) salt) resulting in patterned HSQ.

As shown in FIG. 4E, a gate dielectric layer 520 is deposited on thetransistor. In some embodiments, gate dielectric layer 520 is depositedby chemical vapor deposition (CVD), plasma-enhanced chemical vapordeposition (PECVD), atomic layer deposition (ALD), evaporation, physicalvapor deposition (PVD), chemical solution deposition, or other likeprocesses.

Thereafter, the gate dielectric layer 520 can be etched to expose thethin metal layer 506 in the source region 510 and drain region 512. Thegate dielectric layer 520 can be the same material as the dielectriclayer or different. Gate dielectric layer 520 includes a material with ahigh-k dielectric constant.

The high-k dielectric material(s) can be a dielectric material having adielectric constant greater than 4.0, 7.0, or 10.0. Non-limitingexamples of suitable materials for the high-k dielectric materialinclude oxides, nitrides, oxynitrides, silicates (e.g., metalsilicates), aluminates, titanates, nitrides, or any combination thereof.

Examples of high-k materials include, but are not limited to, metaloxides such as hafnium oxide, hafnium silicon oxide, hafnium siliconoxynitride, lanthanum oxide, lanthanum aluminum oxide, zirconium oxide,zirconium silicon oxide, zirconium silicon oxynitride, tantalum oxide,titanium oxide, barium strontium titanium oxide, barium titanium oxide,strontium titanium oxide, yttrium oxide, aluminum oxide, lead scandiumtantalum oxide, and lead zinc niobate. The high-k material may furtherinclude dopants such as, for example, lanthanum and aluminum.

A gate dielectric can be on the order of 10 to 100 Angstroms thick andcan include a stacked structure. For example, in some embodiments, agate dielectric is on the order of 10 to 90 Angstroms thick. In someembodiments, a gate dielectric is on the order of 10 to 80 Angstromsthick. In some embodiments, a gate dielectric is on the order of 10 to70 Angstroms thick. In some embodiments, a gate dielectric is on theorder of 10 to 60 Angstroms thick. In some embodiments, a gatedielectric is on the order of 10 to 50 Angstroms thick.

FIG. 4F illustrates the transistor after gate electrode 522, sourceelectrode 524, and drain electrode 526 are deposited and patterned onthe transistor. The gate electrode 522, source electrode 524, and drainelectrode 526, can be deposited and patterned by any methods known inthe art. The deposition and patterning of the gate electrode 522, sourceelectrode 524, and drain electrode 526, can be done in a single step orin more than one step, such as two separate steps.

In some embodiments, the gate electrode 522 can be a T-gate, forexample, to lower the gate resistance. Gate electrode 522 can be formedof doped or undoped polysilicon, doped or undoped poly-SiGe, or metal.

In some embodiments, as shown in FIG. 4G, a low-temperature anneal canbe performed. For example, a low temperature anneal can be conducted ata temperature of 400° C. to 500° C. In some embodiments, the lowtemperature anneal can form end-bonded contacts, such as contactsillustrated in FIG. 2B.

The nanomaterial 504 can contact the thin metal layer 506 at each end ofthe area containing the gate electrode 522. A low temperature annealprocess can be performed by conventional methods known in the art. Insome embodiments, an annealing step removes the nanomaterial layer fromthe top of portions of the thin metal layer. For example, carbon, in thecase of graphene, CNT, and related materials, can dissolve into the thinmetal layer during the annealing process upon raising the temperature.

FIG. 4H illustrates the transistor after the resist layer is etched fromthe transistor, thus exposing the thin metal layer 506 between thesource electrode 524 and gate 522 and between the gate 522 and the drainelectrode 526. In some embodiments, the gate dielectric remaining on thetransistor has a length of less than 10 nm, or in some embodiments, lessthan 5 nm.

FIG. 5 depicts an exemplary transfer curve of an embodiment of thedisclosure in which a low-temperature anneal at 400° C. was conducted inan exemplary transistor containing a thin Ni layer and carbon nanotubenanomaterial. The y-axis represents the current passing from source todrain, and the x-axis represents a gate electrode voltage. As is shown,with Ni and a low temperature anneal, the present disclosure can providea device with good transistor activity.

The gate electrode can be any suitable metal. For example, in someembodiments, the gate electrode can include palladium (Pd), aluminum(Al), or gold (Au).

The gate electrode can be formed by either etch process or lift-offprocess. In etch process, metal can be deposited by ALD, CVD, or PVD tocover the full surface. Then, after covering the surface, standardlithography and etch processes used in semiconductor applications can beused to remove metal and leave T-shaped gate structure as shown in FIG.4F 522.

In an exemplary lift-off process, the resist layer can be firstpatterned and developed. Then, after patterning and developing theresist layer, a metal layer can be deposited covering the full surface.Any metal that would not form the final gate structure can be lifted offwith resist during a resist removal process.

As used herein, the terms “invention” or “present invention” arenon-limiting terms and not intended to refer to any single aspect of theparticular invention but encompass all possible aspects as described inthe specification and the claims.

As used herein, the term “about” modifying the quantity of aningredient, component, or reactant of the invention employed refers tovariation in the numerical quantity that can occur, for example, throughtypical measuring and liquid handling procedures used for makingconcentrates or solutions. Furthermore, variation can occur frominadvertent error in measuring procedures, differences in themanufacture, source, or purity of the ingredients employed to make thecompositions or carry out the methods, and the like. In one aspect, theterm “about” means within 10% of the reported numerical value. Inanother aspect, the term “about” means within 5% of the reportednumerical value. Yet, in another aspect, the term “about” means within10, 9, 8, 7, 6, 5, 4, 3, 2, or 1% of the reported numerical value.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A method of fabricating a semiconductor device,the method comprising: depositing a dielectric layer on a substrate,depositing a nanomaterial comprising carbon nanotubes on the dielectriclayer, depositing a thin metal layer on the nanomaterial, formingend-bonded contacts between the thin metal layer and the carbonnanotubes, patterning a resist layer to expose the thin metal layer in asource contact region and a drain contact region; selectively removing aportion of the resist layer and the thin metal layer from a gate area,depositing a gate dielectric layer on the semiconductor device, removingthe gate dielectric layer from the source contact region and the draincontact region, and patterning a gate electrode in the gate area, asource electrode in the source contact region, and a drain electrode inthe drain contact region.
 2. The method of claim 1, the method furthercomprising performing a low-temperature anneal before removing theportion of the resist layer.
 3. The method of claim 2, wherein thelow-temperature anneal is performed at a temperature of 400° C. to 500°C.
 4. The method of claim 1, wherein the semiconductor device comprisesan ungated region with a length of less than 50 nm.
 5. The method ofclaim 4, wherein the semiconductor device comprises an ungated regionwith a length of less than 10 nm.
 6. The method of claim 5, wherein thesemiconductor device comprises an ungated region with a length of lessthan 5 nm.
 7. The method of claim 1, wherein removing the thin metallayer from the gate area comprises wet-etching the thin metal layer. 8.The method of claim 1, wherein the nanomaterial comprises graphene. 9.The method of claim 1, wherein the resist layer comprises hydrogensilsesquioxane.
 10. The method of claim 1, wherein removing the portionof the resist layer comprises wet etching the resist layer.
 11. Themethod of claim 1, wherein the gate dielectric layer is deposited byatomic layer deposition.
 12. The method of claim 1, wherein the gatedielectric is deposited by chemical vapor deposition.
 13. The method ofclaim 1, wherein the thin metal layer comprises Ni, Mo, or Pd.
 14. Themethod of claim 13, wherein the thin metal layer comprises Ni.
 15. Themethod of claim 1, wherein patterning the gate electrode, sourceelectrode, and drain electrode comprises a single step.
 16. The methodof claim 1, further comprising removing the resist layer from above thethin metal layer after patterning the gate electrode in the gate area.17. The method of claim 1, wherein the thin metal layer is deposited toa thickness ranging from 5 nm to 50 nm.
 18. The method of claim 1,wherein patterning the gate electrode comprises patterning a T-gate.